1. Field of the Invention
The present invention generally relates to a method of molding a cart and, more particularly, to a method of molding a cart using high pressure and low pressure molding processes.
2. Background Description
Shopping carts are manufactured using many different assembly methods. For example, carts may be assembled with steel (or other metal) or plastic components. In steel cart assemblies, the assembly process can be time consuming and expensive. This may include welding processes as well bolting and other assemblies. These other assemblies may include tube bending, wire forming, plating and the like. For example, the basket of the cart may have to be formed from wires and welded to form the ribbed pattern of the basket, and may also have to be welded to the frame. In combination with the welding, bolts or rivets may also have to be used in order to properly secure the basket to the frame. Components such as, for example, child seats, trays, handles and the like may also have to be bolted or otherwise fastened to the frame or to the basket, itself. It is also known that steel carts have a tendency to weather badly (e.g., rust) as well as bend and twist due to misuse or large loads placed in the basket. This latter problem may cause the wheels to wobble and may effectively render the cart inoperable or at the very least very difficult to maneuver.
Another example of shopping carts may include plastic baskets or other components mounted to a steel frame. The steel frame will still have the same shortcomings as described above. But in addition to these shortcomings, the basket will have to be mounted to the frame via bolts or other fasteners. In addition, other components may also have to be mounted to either the basket or the shopping cart, expending valuable resources. Also, current molding processes (i.e., injection molding techniques) are limited in application, and are capable of only producing solid molded components which have high molded in stress. This contributes to limited chemical and environmental resistance. Also, these molded components exhibit sink marks especially in thick walls or wall cross sections, and may have increased weight due to the need for increased material to mold the solid components. All of these factors significantly contribute to the manufacturing costs of the shopping cart.
In view of the above and other shortcomings, what is needed is an all plastic shopping cart that does not exhibit sink marks or other defects, and offers extended resistance to environmental and use conditions. The plastic cart should have a smooth wall for an appealing appearance, and should have a high stiffness to weight ratio for structural integrity and stylish appearance. The plastic cart should also be easy to manufacture, exhibiting fewer components and low manufacturing costs. The manufacturing process should also produce low molded in stresses. These plastic carts should also exhibit environmental resistance such as UV, rust and chemical resistance, as well as high structural integrity.
In one aspect of the present invention, a method of molding components of a shopping cart includes plasticizing a resin to a molten form. The resin is then forced into a mold to a mold pressure of approximately 1,500 PSI or less. A blowing agent is then injected into a stream of the molten resin to form a substantially center hollow channel within the molten resin. The molten resin is cooled and a formed substantially hollow plastic component is removed from the mold. A second blowing agent may also be used prior to the forcing step in order to form a plurality of voids within the walls of the substantially hollow plastic component.
In another aspect of the present invention, a blowing agent is injected or blended into the molten resin to form a homogenous mixture. The homogenous mixture is then forced into the mold. The mixture is then cooled and the formed plastic component is removed from the first mold.
In still another embodiment, the molten resin is forced into a mold to create a frame component of the shopping cart. The mold pressure within the mold is approximately greater than 1,500 PSI. The blowing agent is injected into a stream of the molten resin to form a substantially center hollow channel within the frame component.
A combination of these above processes are also contemplated by the present invention. For example, two or more components using any of the above embodiments may be used to form the shopping cart of the present invention. It is also noted that the pressures within the mold may also vary depending on the measurement locations and may include pressure above or below 1,500 PSI.